A Network Address Translation (NAT) device modifies an IP address and port in an IP header when a packet transits across the NAT device. NAT devices are widely deployed at home/enterprise networks and the internet. NAT devices, however, break Voice over Internet Protocol (VoIP) calls.
Some firewalls are configured to block User Datagram Protocol (UDP) and only allow Hypertext Transfer Protocol (HTTP) (TCP 80) or HTTP Secure (HTTPS) (TCP 443) to pass, usually for security reasons. Because voice packets are sent on UDP, firewalls that block UDP also block voice traffic. To summarize, both NAT and firewalls that block UDP can block media communication in VoIP and result in one-way voice or no voice.
It would therefore be desirable to provide an improved NAT/Firewall traversal.